1. Field of the Invention
The present invention relates to a fuel injection device for use in an internal combustion engine.
2. Description of the Related Art
The present applicants have proposed a unit injector comprising a plunger driven by an engine, a high pressure fuel chamber filled with fuel which is pressurized by the plunger, a needle moved in accordance with the fuel pressure in the high pressure fuel chamber to open a valve opening when the fuel pressure exceeds a predetermined pressure, a spill valve inserted slidably in a bore to control the spillage of fuel in the high pressure fuel chamber, and a piezoelectric element which moves the spill valve axially relative to the bore and controls the opening and closing of the spill valve, wherein the fuel injection is performed when the spill valve is closed (See copending U.S. patent application No. 284,434 or copending British Patent Application No. 8827575.3)
In this unit injector, the end of the bore is open to a fuel spill chamber having an increased diameter, and a valve seat is formed on the end of the bore. The spill valve has an enlarged head portion positioned in the fuel spill chamber and able to be seated on the valve seat. In addition, the spill valve has an annular fitting portion formed at an end thereof opposite to the enlarged head portion and in tight contact with the inner circumferential wall of the bore. A high pressure fuel introduction chamber is formed around the outer circumferential wall of the spill valve between the annular fitting portion and the enlarged head portion, and connected to the high pressure fuel chamber, and a spring is mounted on the spill valve to bias the spill valve in the open direction.
The fuel injection is started by closing the spill valve against the spring, and when the enlarged head portion is moved away from the valve seat, the high pressure fuel in the high pressure fuel chamber is spilled out into the fuel spill chamber via the high pressure fuel introduction chamber, and thus the fuel injection is stopped. In this unit injector, however, when the spill valve is opened to stop the fuel injection, since the high pressure fuel is spilled out into the fuel spill chamber, the pressure of the fuel in the fuel spill chamber is temporarily high, and at this time, since this high pressure acts on the tip face of the enlarged head portion of the spill valve and provides a force pushing the spill valve in the closed direction, the spill valve is closed again almost as soon as it is opened. As a result, problems arise in that the fuel cannot be appropriately injected, and in particular, a good fuel cutting operation of the fuel injection cannot be obtained.